Condensed answer: A front BMX hub can be installed on the front wheel of a fixie. The procedure will more than likely require new spokes of a different length.
A rear BMX hub is narrower than a dedicated rear track/fixed-gear hub and cannot be installed on a fixed-gear bike without spacers to make up for the gaps.
BMX and Fixed-Gear Hub Measurements
The main hub measurement is known as Over Locknut Dimension (O.L.D.) and refers to the usable part of the hub or the distance between the outer sides of the two locknuts.
The O.L.D. of front and rear BMX and fixed-gear hubs is presented in the table below:
|BMX (female hub)
|BMX (male hub)
The data reveals the following:
1. The rear O.L.D. of fixed-gear hubs is longer than that of BMX hubs.
2. The front O.L.D. of fixed-gear hubs is equal to that of BMX front hubs.
Installing a Rear Male BMX Hub On a Fixed-Gear Bike
Strictly speaking, a rear BMX hub isn’t compatible with a track frame due to the different spacing.
However, there is a way to circumvent the issue. The solution is not perfect, and some people won’t like it, but it’s the only option.
You can put a rear BMX hub on a fixed-gear by filling the gaps with spacers. A standard male rear BMX hub has a 14mm axle and would require two 5mm spacers with 14mm entrances to fill the gaps.
What about a female hub?
Technically, the same procedure can be done with a female hub. The only difference is that the spacers will have to be placed on the female hub bolts.
However, the spacers will weaken the hub’s composure because less of the bolt’s body will be in the axle.
Therefore, it’s not recommended to do this experiment with a rear female hub.
The O.L.D. difference between a fixed-gear and a BMX rear hub is 10mm (120mm-110mm).
Meanwhile, the total length of a hub bolt is about 30mm. The threaded part is even less than that.
If the gap that a BMX hub leaves when installed on a fixed-gear frame is roughly 10mm, only 20mm of the bolt will go into the axle. This isn’t enough to ensure maximum stability.
Additional Downsides Of Installing a Rear BMX Hub On a Fixie
- No longer fixed-gear
BMX bikes are single-speed rather than fixed-gear. Or in other words, they have only one gear, but the bike can coast (the rear wheel can move independently of the drivetrain).
This is achieved either via a freewheel threaded onto the rear hub оr via a single-speed cassette hub.
Of course, one can thread a fixed-gear cog onto a BMX hub, but the system will not be secure. Riding with such a contraption will be dangerous.
The image above shows a dedicated fixed-gear hub. Notice that the left side has one threaded section while the left has two (a.k.a. stepped threading).
The left side is designed for a freewheel cog while the right is designed for a fixed-gear cog. If you want your bike to be a single-speed machine, you should use a freewheel on the left side.
If you want it to be a fixie, you will have to install a fixed gear cog in conjunction with a lock ring on the right side.
The large thread is designed for а fixed-gear cog while the smaller one accepts the lock ring. Both have different thread directions. The cog thread is right-hand while the lock ring thread is left-hand.
The purpose of this threading is to ensure that the cog doesn’t unscrew when the rider stops via the pedals.
When the rider is pedaling forward, the chain is also spinning the cog in a clockwise direction and therefore tightening it to the hub.
However, when the rider back-pedals, the chain spins the cog anti-clockwise. Meanwhile, the rear wheel wants to continue spinning forward due to inertia. As a result, it becomes possible for the cog to untighten itself.
To prevent this outcome, a lockring with a reverse thread is used to secure the cog. The lockring goes onto the small thread.
When the rider backpedals, the pressure on the cog is anti-clockwise. If the cog gets loose and tries to rotate, it will spin the lockring too. But since the lockring is reverse-threaded, the untightening motion of the cog tightens the lockring. As a result, the cog is always secure.
Some people may try to secure the cog onto the freewheel hub via a lock ring (usually taken from an old-school bottom bracket). While this method adds security, it’s not enough because the lockring will have the same threading as the cog and can untighten along with it.
For maximum safety, it’s recommended to use a freewheel cog rather than a fixed one.
Installing a Front BMX Hub On a Fixie
A quick-release front road/track hub has the same spacing as a BMX hub – 100mm. Therefore, a BMX front hub will fit right away.
If the fork is made for a thru-axle, the conversion will fail because thru-axles are thicker and have a different structure than what a BMX hub is designed for.
Note: More than likely, the conversion will also require a new set of spokes as the current ones could be too short.
The Advantages and Disadvantages of Installing a BMX Hub On a Fixie
Truth be told, the benefits of installing a BMX hub on a fixie are questionable. The main two are durability (BMX hubs are strong) and the ability to save money if you already have a hub on hand.
That said, those are not enough to outweigh the downsides of the procedure which are:
- No longer fixed gear (as already mentioned, a BMX hub cannot be used safely with a fixed gear cog.)
- Incompatibility with the frame (а rear BMX hub is 10mm narrower than a dedicated track model).